Ice accretion is a problem which is known to affect aircraft gas turbine engines during flight. Ice crystals which form at high altitudes are drawn into the aircraft engines and are deposited on internal components, in particular compressor components, of the engines. When the engine thrust is relatively high, for example during climb and cruise conditions, the temperature within the engine and the high velocity airflow through the engine prevent ice crystals from accumulating within the compressor. However, when the engine thrust is low, for example during descent of the aircraft, the combination of low temperatures and low velocity air allows ice to build up to significant thicknesses on the compressor components.
Ice shedding occurs when there is a subsequent increase in the temperature of the engine, for example as a consequence of an increase in engine power at the end of the descent phase, or because of an increase in the ambient air temperature as the aircraft descends. In addition, ice shedding may occur simply because the amount of ice accretion reaches a point at which the drag acting on the accreted ice exceeds the strength of its adhesion to the components.
Shed ice passes through the engine which can cause damage to downstream engine components.
All engines on a multi-engine aircraft are generally subjected to the common weather conditions at some point in operation. Therefore, ice crystal accretion and subsequent shedding is recognised as “common mode” threat which can potentially affect all engines of the aircraft simultaneously. A common mode threat reduces the inherent redundancy and hence increased safety associated with multiple engined aircraft.
It is normal practice for a pilot to increase the speed of an engine intermittently during descent in order to raise the temperature of the engine thereby preventing substantial ice crystal accretion. Increasing the speed of the engine has two disadvantages. Firstly, the thrust will increase beyond that required by the aircraft to descend, making descent times longer. Secondly, it causes more fuel to be burnt during descent.
Alternatively, or in addition, components of the engine which are vulnerable to damage by shed ice are strengthened to avoid catastrophic damage. Strengthening engine components creates an increase in weight of the engine or an increase in drag, thus reducing engine efficiency and increasing the rate of fuel consumption.